Understanding the genetics of C3 photosynthesis, particularly its regulation, is essential to undertake photosynthetic improvement. The expression of the photosynthesis-associated genes is regulated at different levels (transcriptional, post-transcriptional, post-translational), but very little is known about the regulatory networks involved. This chapter introduces the photosynthesis-associated core genes encoded either in the nucleus or in the chloroplast and discusses how different internal (e.g. redox state, circadian rhythm) and external (e.g. abiotic stresses, light) signals regulate their transcription, particularly in crop plants. Since the molecular mechanisms underlying the regulation of photosynthesis-associated genes is poorly understood, this chapter also discusses what is known regarding the transcriptional regulation of photosynthesis in C3 crops, mainly rice and tomato. Among the regulators described, few were shown in the field to have the potential to improve photosynthesis. How the state-of-the-art knowledge can be used for photosynthesis improvement and future work perspectives, including the use of transplastomics, is also discussed.